ASoC: atmel-ssc: remove clk_disable_unprepare call from critical section

clk_prepare/unprepare (and indirectly clk_prepare_enable/disable_unprepare)
may sleep and thus cannot be called in critical section.

This patch fix a bug introduced by commit
6f0d9479 where clk_disable_unprepare was
called with user_lock hold.

Signed-off-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Mark Brown <broonie@linaro.org>